Many of the behavioral effects of testosterone occur after conversion to estradiol by the aromatase enzyme. Although there has been interest in the role of aromatase as a regulator of male mating behavior, relatively little research has considered whether aromatase regulates other social behaviors. This application proposes to examine how aromatase mediates the effects of testosterone on paternal behavior and inter-male aggression in the monogamous California mouse, a species in which males show high levels of both paternal behavior and aggression. First, the proposed research will test whether the positive effect of testosterone on paternal behavior in this species occurs through conversion to estradiol. Second, this research will examine whether increased testosterone after winning an aggressive encounter increases aggression, and whether aromatase mediates such an effect. Finally, this research will determine if central aromatase activity is correlated with paternal and aggressive experience. Understanding how aromatase modulates behavioral effects of T has potentially important implications both for our understanding of the neuroendocrine control of mammalian social behavior and for human health. Testosterone replacement therapy in humans is becoming increasingly common, and most studies restrict behavioral analyses to sexual functions, mood, and cognition. In summary, this proposal examines how aromatase mediates the effects of testosterone and social behavior.